In recent years, automotive emission control has been tightened for global environment protection. Under such circumstances, improving the fuel efficiency of vehicles has been an important issue, and strengthening and sheet metal thinning of automotive parts have been required. A highly effective way of obtaining a higher-strength and thinner-sheet-metal automotive part is to press form a steel sheet having predetermined strength as a raw material of the automotive part into a part of a predetermined shape. However, when the steel sheet has higher strength, its press formability is deteriorated, and forming the steel sheet into the desired part shape is more difficult.
The following technique is known to solve the above-mentioned problem: while hot press forming a raw material steel sheet heated to high temperature into a desired shape using a press mold, the steel sheet is quenched in the press mold by releasing heat, thus enhancing the strength of the hot press formed part. For example, Patent Literature (PTL) 1 proposes a technique in which, when manufacturing a part of a predetermined shape by hot pressing a blank sheet (steel sheet) heated to an austenite single phase region of about 900° C., the blank sheet is quenched in a press mold simultaneously with the hot press forming, thus enhancing the strength of the part.
However, the technique proposed in PTL 1 has a problem in that, when heating the steel sheet to high temperature of about 900° C. before the pressing, oxide scale (iron oxide) forms on the surface of the steel sheet, and the oxide scale peels during the hot press forming and damages the press mold or the surface of the hot press formed part. Besides, the oxide scale remaining on the surface of the part causes poor appearance and lower paint adhesion. Accordingly, the oxide scale on the surface of the part is typically removed by a process such as pickling or shot blasting. Such a process, however, causes lower productivity. Furthermore, while suspension parts of vehicles, automotive body structural parts, and the like are also required to have excellent corrosion resistance, the corrosion resistance of the hot press formed part by the technique proposed in PTL 1 is insufficient because a rust preventive film such as a coating layer is not provided on the raw material steel sheet.
For these reasons, there is demand for hot press forming techniques that can suppress the generation of oxide scale during heating before hot press forming and also improve the corrosion resistance of the hot press formed part. To meet this demand, coated steel sheets having films such as coating layers on their surfaces, hot press forming methods using coated steel sheets, etc. are proposed.
For example, PTL 2 proposes a technique in which a steel sheet coated with Zn (zinc) or a zinc-based alloy is heated to 700° C. to 1200° C. and then hot press formed to obtain a hot press formed part having a Zn—Fe-based compound or a Zn—Fe—Al-based compound on its surface. PTL 2 describes that the use of the steel sheet coated with Zn or a Zn-based alloy suppresses the oxidation of the surface of the steel sheet during heating before hot press forming, and also enables a hot press formed part having excellent corrosion resistance to be obtained.
With the technique proposed in PTL 2, the generation of oxide scale on the surface of the hot press formed part is suppressed to some extent. However, Zn in the coating layer may cause liquid metal embrittlement cracking, resulting in cracks of about 100 μm in depth in the surface layer part of the hot press formed part. Such cracks pose various problems such as a decrease in fatigue resistance of the hot press formed part.
In view of this problem, PTL 3 proposes a method in which, in the case of manufacturing a hot press formed article by hot press forming a coated steel sheet obtained by providing a Zn—Fe-based coating layer on the surface of a steel sheet, the coated steel sheet is heated to a temperature not less than the Ac1 transformation temperature of the steel sheet and not more than 950° C. and then cooled to a temperature not more than the congealing point of the coating layer, before starting the forming. PTL 3 describes that liquid metal embrittlement can be suppressed by starting the forming after the coated steel sheet is cooled to the temperature not more than the congealing point of the coating layer.